Flat display devices such as liquid crystal displays are widely used in various consumer electronic products. In a case that the flat display devices are applied to portable electronic devices, it is a very important issue to reduce the power consumption.
A liquid crystal device is a flat display device that uses the light modulating properties of liquid crystal molecules. By changing an electric field, the orientation of the liquid crystal molecules is changed. Consequently, the polarization of the incident light is changed to produce a corresponding image. That is, for displaying the contents of the image on the flat display device, the image frame is shown on the display panel according to the polarity inversion of the data lines or the pixel units.
FIGS. 1A˜1C are schematic circuit diagrams illustrating a control circuit for controlling the operations of a display panel according to the prior art. For clarification, the pixel units of the display panel are not shown in the following drawings, and only some of the data lines are shown. The other components (e.g. the polarity control lines and the timing controller) of the display panel are not described herein.
For clarification and brevity, only four data lines and four voltage driving units are shown in FIGS. 1A˜1C. From top to bottom of the left side of these drawings, the control circuit includes a first voltage driving unit 111, a second voltage driving unit 112, a third voltage driving unit 113, and a fourth voltage driving unit 114. Moreover, from top to bottom of the right side of these drawings, the control circuit includes a first data line 101, a second data line 102, a third data line 103, and a fourth data line 104.
Moreover, a first period before the polarity inversion includes a first stage I and a second stage II, and a second period after the polarity inversion includes a third stage III.
As shown in FIG. 1A, the display panel is operated in the first stage I. During the first stage I, the first data line 101 and the third data line 103 are connected with the first voltage driving unit 111 and the third voltage driving unit 113, respectively. Both of the first voltage driving unit 111 and the third voltage driving unit 113 provide positive driving voltages. On the other hand, the second data line 102 and the fourth data line 104 are connected with the second voltage driving unit 112 and the fourth voltage driving unit 114, respectively. Both of the second voltage driving unit 112 and the fourth voltage driving unit 114 provide negative driving voltages.
During the process of controlling the display panel, the polarities providing to the pixel units by respective data lines may be switched between positive polarities and negative polarities.
As shown in FIG. 1B, the display panel is operated in the second stage II. Before the polarity inversion, these data lines are connected with a ground voltage. Consequently, the voltages of all data lines are 0 volt in the second stage II.
As shown in FIG. 1C, the display panel is operated in the third stage III. The connections between the data lines and the voltage driving units in the third stage III are different from the ones in the first stage I. In the third stage III, the data lines and the voltage driving units are connected with each other in a staggered arrangement.
That is, as shown in FIG. 1C, the first data line 101 and the third data line 103 are connected with the second voltage driving unit 112 and the fourth voltage driving unit 114, respectively. Both of the second voltage driving unit 112 and the fourth voltage driving unit 114 provide negative driving voltages. On the other hand, the second data line 102 and the fourth data line 104 are connected with the first voltage driving unit 111 and the third voltage driving unit 113, respectively. Both of the first voltage driving unit 111 and the third voltage driving unit 113 provide positive driving voltages.
FIG. 1D is a schematic timing waveform diagram illustrating associated voltages of the first data line in different stages according to FIGS. 1A˜1C. In the first stage I, since the first data line 101 is electrically connected with the first voltage driving unit 111, which provides the positive driving voltage (e.g. +5V), the voltage applied to the first data line 101 is maintained at +5V.
For the subsequent polarity inversion, the data lines with the positive driving voltage are discharged. That is, in the second stage II, the first data line 101 is connected with the ground voltage. Consequently, the voltage of the first data line 101 is 0 volt.
In the third stage III, since the first data line 101 is electrically connected with the second voltage driving unit 112, which provides the negative driving voltage (e.g. −5V), the voltage applied to the first data line 101 is decreased to −5V.
As shown in FIG. 1D, in the beginning of the third stage III, the magnitude of the current flowing through the first data line 101 is abruptly changed and a large transient negative current is generated. Consequently, the overall average current value is increased and the power consumption is increased. After the third stage III, the polarity inversion is repeatedly performed. Before the polarity inversion, all data lines should be connected with the ground voltage too. Then, the driving voltages with different polarities are provided to the data lines.
FIG. 1E is a schematic timing waveform diagram illustrating associated voltages of the second data line in different stages according to FIGS. 1A˜1C. Since the first data line 101 and the second data line 102 are connected with the voltage driving units with different polarities in the first stage I and the third stage III, the voltage change of FIG. 1E and the voltage change of FIG. 1D are opposite.
Similarly, as shown in FIG. 1E, in the end of the second stage II and in the beginning of the third stage III, the magnitude of the current flowing through the second data line 102 is abruptly changed and a large transient positive current is generated. Consequently, the overall average current value is increased and the power consumption is increased.
From the above discussions in FIGS. 1A˜1E, the driving voltage of the data line is switched back and forth between the positive polarity and the negative polarity. Before each polarity inversion, all data lines should be connected with the ground voltage. In such way, the driving voltage provided by the voltage driving unit should increase or decrease the voltage of the data line from 0 volt during each polarity inversion. Under this circumstance, the electric energy is gratuitously wasted.